In recent years tremendous progress has been made in the design and synthesis of peptides with high selectivities for the different types (mu, delta, kappa) of opioid receptors. However, a major obstacle to the development of these synthetic opioid peptides as clinically useful therapeutic agents has been their low permeability through biological barriers (e.g., intestinal mucosa, blood brain barrier). Unfortunately, some of the structural features of an opioid peptide [e.g., free N- terminal amino and C-terminal carboxyl groups and side chain carboxyl (e.g., Asp, Glu) and amino (e.g., Lys, Arg) groups] that bestow affinity and specificity of the molecule for the different opioid receptors, also bestow on the molecule undesirable physicochemical properties which limit its membrane permeability. The objectives of this project are to synthesize and biologically evaluate cyclic prodrugs of opioid peptides which will transiently mask these undesirable physicochemical properties, thereby enhancing their membrane permeability. The novel prodrug strategy employed in this study takes advantage of an esterase sensitive system as a linker to convert linear opioid peptides to cyclic prodrugs. These cyclic prodrugs will decrease the polarity and size of the peptide and restrict its conformational freedom, thus, enhancing its membrane permeability. Through the masking of one or both of the terminal ends of an opioid peptide, the propensity of the peptide to be degraded by exo and endo peptidase should also be reduced. In addition, by employing an enzyme trigger (esterase) to release the peptide, a sustained release system phenomena may occur resulting in an increased biological half- life. To properly evaluate this prodrug system, various opioid peptides with high selectivities for different types of opioid receptors and unique structural features have been selected and cyclic prodrugs of these peptides will be synthesized. Sensitive and selective analytical methods for the linear and cyclic peptides will be developed and used to determine their physicochemical properties (e.g., partition coefficients) and their disposition in biological fluids and tissues. The chemical and enzymatic (e.g., esterase, protease) stability of these cyclic prodrugs will be evaluated in vitro. Experiments have been designed to evaluate the receptor binding activity of the cyclic prodrugs and their ability to elicit pharmacological effects in vivo. Finally, the permeability of these linear and cyclic opioid peptides through the intestinal mucosa (an in situ rat intestinal perfusion model) and through the blood brain barrier (an in situ rat brain perfusion model) will be determined. If deemed necessary, -cell culture models of the intestinal mucosa and the blood brain barrier will be used to determine intrinsic permeabilities and elucidate pathways by which the opioid peptides and their cyclic prodrug penetrate these biological barriers. The results of this study could provide medicinal chemists with a generally applicable prodrug system for enhancing the membrane permeability of opioid peptides.